1. Field of the Invention
The present invention relates to a semiconductor memory device, and in particular to a structure for reducing a current consumption in a data holding mode for holding data stored in a dynamic memory cell.
2. Description of the Background Art
In a dynamic semiconductor memory device (which will be referred as a DRAM), a memory cell of one bit is formed of one MOS transistor (insulated gate field-effect transistor) and one capacitor. The DRAM has been widely used as a storage device of a large storage capacity, because an area occupied by a memory cell of one bit is smaller and a cost per bit is lower as compared to a static random access memory (SRAM) in which a memory cell of one bit requires a plurality of transistor elements.
The DRAM stores information in a form of electric charges. In order to prevent destruction of storage data which may be caused by reduction in quantity of the accumulated charges due to current leakage, it is necessary to perform refreshing for periodically reading and rewriting storage data from and into the memory cell.
In recent years, DRAMs have been widely used as main storage devices of portable terminals such as a laptop computer. Information equipments such as a portable terminal utilize a battery as an operation power source, and it is necessary to minimize a current consumption of the portable terminal in order to maximize a battery life. Even during a period in which information processing is not performed in the portable terminal, it is necessary to refresh periodically storage data of the DRAM. Such an operation mode is called "data holding mode" that only the refreshing of storage data of the memory cell is internally performed without performing input/output of data to and from the DRAM as described above.
In connection with this data holding mode, it has been attempted to reduce a current consumption, for example, by lowering a power supply voltage of the DRAM and/or increasing a length of period between refresh operations.
However, in order to increase the battery life in the battery-powered equipments, it is now required to reduce further the data holding current consumed in the data holding mode (i.e., current during the refreshing and current during standby).